The present invention relates to a navigation apparatus for an automobile which displays the present position of a vehicle on a road map together with data necessary while driving, to visually observe one's position on a map.
A navigation apparatus which detects the present position of a vehicle and displays it on a road map, and data necessary for driving is known from Japanese Patent Laid-Open No. 206817/1982 entitled "Automobile Navigator", for example. This prior-art apparatus has the following construction. Map data for the display of a road map of a specific zone and map data for a plurality of zones adjacent to that specific zone are stored in a memory means, and when the present position of the vehicle calculated on the basis of signals from a distance detection means and a direction detection means is judged to have reached a predetermined adjacent zone, the map data corresponding to that adjacent zone is read out from the memory means so that the road map corresponding to the map data is updated and displayed on a display means, so that a display of the road map of the specific zone can be automatically changed over to a display of the road map of the adjacent zone corresponding to the present position of the vehicle.
However, since the display means (for example, CRT) of the navigator must be installed within the vehicle, it must be rather small (for example, 6 inches: 9 cm long and 12 cm wide), and a map at a scale of 1/100,000 must be displayed on the CRT. If the vehicle travels at 60 km/hr, the present position of the vehicle on the CRT moves at a rate of 1 cm/min. This means that it takes between about 9 to 12 minutes for the present position to move from one end to the other end of the surface of the CRT. The map data is updated by judging that the present position based on the movement of the vehicle has reached a predetermined adjacent zone, and then reading out map data corresponding to that adjacent zone from the memory means. About one minute is needed for reading the map data out of the memory means and, in addition, a tape must be searched for the position of the map data corresponding to that adjacent zone before the map data can be read. If the time required for completing the update of the map data after the arrival of the present position at the predetermined adjacent position is between about 2 to 4 minutes in this way, the period during which the present position is displayed on the screen surface comes to be shorter when the vehicle is traveling at high speed, so that the update speed of the map data is delayed or slow when the usage time of the map data is short, etc., and a sufficient navigation function can not be exhibited because the display shifts to the next picture without displaying the present position on the screen surface adequately.